Porous polymer bodies having many communicating micropores are used in various fields as separator membranes for use in ultrapure water production, purification of chemical solutions, and water treatment; waterproof moisture-permeable films for use in clothing and sanitary supplies; and battery separators for use in secondary batteries.
Secondary batteries are widely used as power supplies for portable devices, such as OA, FA, household electrical appliances, and communication devices. In particular, portable devices that include lithium-ion secondary batteries are becoming widespread because the lithium-ion secondary batteries have high volumetric efficiency and can reduce the size and weight of the devices. Large secondary batteries are under research and development in many fields related to energy and environmental issues, including load-leveling, UPSs, and electric vehicles. Lithium-ion secondary batteries, which belong to one type of nonaqueous electrolyte secondary batteries, are becoming widespread because of their large capacities, high output power, high voltages, and high long-term storage stability.
The lithium-ion secondary batteries are generally designed to have the highest working voltage in the range of 4.1 to 4.2 V. Aqueous solutions are electrolyzed at such a high voltage and cannot be used as electrolyte solutions. Thus, nonaqueous electrolytes, which contain organic solvents, are used as electrolyte solutions that can withstand high voltages. High-dielectric-constant organic solvents, which can dissolve many lithium ions, are used as solvents for nonaqueous electrolytes. Organic carbonate compounds, such as propylene carbonate and ethylene carbonate, are mainly used as high-dielectric-constant organic solvents. A reactive electrolyte, such as lithium hexafluorophosphate, dissolved in a solvent is used as a supporting electrolyte, which serves as a lithium ion source.
Lithium-ion secondary batteries include a separator between a positive electrode and a negative electrode in order to prevent internal short-circuits. From the nature of the system, the separator must have insulating properties. The separator must have a micropore structure in order to achieve high permeability for passage of lithium ions and to diffuse and retain an electrolyte solution. To satisfy these requirements, porous films are used as separators.
With increasing capacity of batteries, separators are becoming more important for battery safety. The characteristics of battery separators that contribute to greater battery safety include shutdown characteristics (hereinafter referred to as “SD characteristics”). Because of the SD characteristics, micropores of a porous film are closed at a high temperature in the range of approximately 100° C. to 150° C. This can intercept ionic conduction in the battery and thereby prevent a subsequent temperature rise in the battery. The lowest temperature at which micropores of a porous film are closed is referred to as the shutdown temperature (hereinafter referred to as the “SD temperature”). Porous films to be used as battery separators must have the SD characteristics.
However, because of recent increases in energy density and capacity of lithium-ion secondary batteries, there have been accidents in which the shutdown function has not worked well, and the internal temperature of a battery exceeded the melting point (approximately 130° C.) of a polyethylene used as a material of a battery separator. This caused thermal shrinkage and rupture of the separator and a short circuit between the electrodes, resulting in ignition. Thus, in order to ensure battery safety, there is a demand for separators having higher heat resistance than that for the present SD characteristics.
To satisfy the demand, a multilayer porous film is proposed that includes a porous layer on at least one surface of a porous polyolefin resin film (Patent Literatures 1 to 5). The porous layer contains inorganic fine particles, for example, of a metal oxide and a resin binder. In these multilayer porous films, a coated layer filled with a large number of inorganic fine particles, for example, of α-alumina is formed on a porous film. In the case of abnormal heat generation and a temperature rise above the SD temperature, the coated layer can prevent a short circuit between the electrodes and significantly improve battery safety.
In particular, in Patent Literature 4, the inorganic fine particles are plate-like particles in order to prevent internal short-circuits and to ensure battery safety. In Patent Literature 5, filler particles having a circularity distribution closer to a perfect circle are used to maintain the porosity of the porous layer.